Feedback circuit for electromagnetic control of photomultiplier tube



July 27, 1965 v. G. SHAW 3,197,642

- FEEDBACK CIRCUIT FOR ELECTROMAGNETIC CONTROL OF PHOTOMULTIPLIER TUBE Filed Nov. 20. 1961 INVENTOR. VINCENT 6. SHAW Attorneys United States Patent 3,197,642 FEEDBACK CIRQUIT FOR ELECTROMAGNETI CONTRQL 8F HHGTOMULTPLIER TUBE Vincent G. Shaw, Unity Township, Westmoreland County, Pa, assignor to Latronics Corporation, La-

trobe, Pa., a corporation of Pennsylvania Filed Nov. 26, 1961, Ser. No. 153,575 3 Claims. (8i. il--2ti7) This invention relates to a photomultiplier tube circuit and more particularly to a circuit for controlling the current in the tube.

Photomultiplier tubes of conventional design are generally capable of multiplying electrons emitted from a cathode by incorporating a number of dynodes between the cathode and the plate. The radiation incident on the tube cathode causes electrons to flow from the cathode in direct proportion to the intensity of the incident illumination. Oftentimes, however, it has been desired to regulate the flow of electrons through the tube in order to prevent tube fatigue caused by excessive electron emission from the cathode in response to high intensity illumination. 7

More particularly, in a photo pyrometric apparatus as disclosed in my copending application Serial No. 725,550, filed April 1, 1958, now Patent No. 3,006,242, the instrument may at times be relatively close to a large mass of intensively heated metal where it is difficult to reduce the amount of radiant energy to the photomultiplier tube to an amount where it does not interfere with the accuracy of response of the instrument to the diflerence in energy in the successive rapidly changing wave length regions. The present invention provides a means for controlling tube current by electromagnetically suppressing the elfect of superfluous electron emission. This is accomplished by the provision of means associated with the photomultiplier tube for electrically suppressing the flow of electrons between the anode and cathode by diverting a portion of the electron flow to a controlled extent.

It is an object of the present invention to provide a photomultiplier tube circuit which regulates the current flow through the tube.

Another object is to provide a feedback circuit for controlling tube current.

A complete understanding of the invention may be had from the following detailed description of a specific embodiment thereof when read in conjunction with the appended drawing, wherein;

The figure is a schematic view of the photomultiplier tube circuit.

Referring to the drawing, there is shown in the figure, which represents a preferred embodiment, a photomultiplier tube generally designated as numeral 11. Connected to the tube cathode 12 is a negative terminal 13 of a suitable source of high voltage sufdcient to operate the tube 11. Also connected to the negative terminal 13 of the power supply is a resistor network 14 which is in parallel with the tube 11 across the dynodes 15 to furnish them with correct operating potentials. The resistor network 14, which is a voltage divider network, is connected to ground point 16 through a suitable resistor 17.

The tube 11 has a cathode 12, a plate 1% and a plurality of dynodes 15 intermediate the cathode and anode. Electrons emitted by the cathode 12 are multiplied by the dynodes 15 and collected on the plate 18, the latter being connected to ground through a plate load resistor 22. As current flows through the tube 11, a negative voltage with respect to ground is developed across the lee resistor 22, which voltage is proportional to the current and is felt at the junction point 21.

In accordance with my invention a variable magnetic field is generated in the region of the cathode 12 of the tube 11 so that electrons emitted by the cathode must pass through the field. A coil 37 is positioned around the envelope of the tube 11 in a location where it will develop a field intermediate the cathode l2 and first dynode, and is oriented with respect to the path of emitted electrons so that the field developed is operative to deflect, to a greater or lesser extent, electrons passing through the field. The field strength is varied in accordance with the signal generated at the plate 18. When the generated signal is too large, as the signal tends to increase the strength of the magnetic field is increased by increasing the current flow in the coil 37. The electrons emitted by the cathode 12 must pass through the field and are deflected from their cathode to anode path to a degree determined by the field strength, more electrons being deflected as the field strength is increased. Conversely, when the signal generated at the anode 18 is relatively Weak, the current in the coil 37 is decreased to lessen the magnetic field strength, thus deflecting fewer electrons.

in circuit with the tube 11 there is a feedback loop, designated generally as 23, which embodies a new circuit for the control of tube current. It comprises a non-inverting amplifier 24 receiving a voltage input from the junction point 21. The output of this amplifier is impressed on the grid 25 of a first triode 26. The triode 26 has its plate 32 connected to a positive terminal 36 of a high voltage power supply'through a suitable load resistance 32a.

There is a voltage divider network 28 comprising resistors 29 and 31 connected to a suitable power supply indicated by terminals 13+ and B. The cathode 27 of the triode 26 is connected to the positive side of resistor 31 so that the cathode 27 is positively biased.

There is a second triode 34 which has its plate 35 directly connected to the positive terminal 36 of the high voltage power supply and its grid 33 directly coupled to the plate 32 of the triode 26. The cathode 38 of the triode 34 is connected to the positive side of resistor 29 through the coil 37 so that the cathode 38 is also positively biased. The electromagnetic coil 37 is in series with the cathode 38 having a connection at point 41 to the positive side of resistor 2? and a connection at 39 to the cathode 38 to complete the feedback circuit. At low photomultiplier tube currents the triode 26 is normally conducting while the triode 34 may be either cut off or conducting at a low reference current level so that none or very little current flows through the coil 37.

As before stated, the nagative output of the non-inverting amplifier 24 is impressed on the grid 25 of the triode 2-6. As this negative voltage increases, the triode 26 conducts less, thereby increasing the potential on the plate 32 which is coupled to the grid 33 of triode 35 causing the latter to increase conduction, thereby increasing the current in the coil 37. As current in the coil increases the magnetic field increases to deflect more (a) a photomultiplier tube having an anode, a cathode and a plurality of dynodes intermediate the anode and cathode,

(b) a high voltage power supply having its negative terminal connected to the cathode of the tube,

(c) a load resistor in the anode circuit of the tube, the anode of the tube being connected to ground through the load resistor,

(d) a non-inverting amplifier connected to receive an input from the anode of the tube,

(c) a first triode connected to receive the output of the amplifier on its grid,

(f) a load resistor in the plate circuit of the first triode, connecting the plate of the first triode to the positive terminal of the high voltage power supply,

(g) a second triode having its plate connected to the positive terminal of the power supply and its grid connected to the plate of the first triode, and

(h) an electromagnetic coil in the cathode circuit of the second triode, the coil being positioned about the photomultiplier tube in the region of the cathode of the tube to develop a magnetic field intermediate the cathode and first dynode of the tube, whereby an increase in electron emission from the cathode of the tube, above a predetermined level, will cause the electromagnetic coil to be operative to deflect a substantial portion of emitted electrons, thereby controlling the amount of current through the tube.

2. The combination as defined in claim 1, including a voltage divider network for biasing the cathodes of the first and second triodes.

3. In a photomultiplier tube circuit, in combination,

(a) a photofultiplier tube having an anode, a cathode, and a plurality of dynodes intermediate the cathode and anode,

(b) a high voltage power supply having its negative terminal connected to the cathode of said tube,

(0) a load resistor in the anode circuit of the tube connecting the anode to ground,

((1) a first triode having its grid connected to the anode of the photomultiplier tube,

(e) a load resistor in the plate circuit of the first triode connecting the plate to the positive terminal of the high voltage power supply,

(f) a second triode having its plate connected to the positive terminal of the power supply and its grid connected to the plate of the first triode, and

(g) an electromagnetic coil in the cathode circuit of the second triode, the coil being positioned about the photomultiplier tube in the region of the cathode of the tube to develop a magnetic field intermediate the cathode and first dynode of the tube,

whereby an increase in electron emission above a predetermined level will cause the electromagnetic coil to be operative to deflect a substantial portion of emitted electrons, thereby controlling the current through the tube.

References (:ited by the Examiner UNITED STATES PATENTS 2,342,986 2/44 Van den Bosch 250-207 2,702,865 2/55 Herzog 250-207 2,707,238 4/55 Fromm 250-207 2,772,368 11/56 Scherbatskoy 250-207 2,840,720 6/58 Van Rennes 250-207 2,850,644 9/58 Parsons 250-207 2,927,502 3/60 Watrous 8822.5 2,994,782 8/61 Gouyon et a1. 250-207 3,006,242 10/61 Shaw 88-225 3,149,235 9/64 Clark 250-207 X RALPH G. NILSON, Primary Examiner.

FREDERICK M. STRADER, Examiner. 

1. IN A PHOTOMULTIPLIER TUBE CIRCUIT, IN COMBINATION (A) A PHOTOMULTIPLIER TUBE HAVING AN ANODE, A CATHODE AND A PLURALITY OF DYNODES INTERMEDIATE THE ANODE AND CATHODE, (B) A HIGH VOLTAGE POWER SUPPLY HAVING ITS NEGATIVE TERMINAL CONNECTED TO THE CATHODE OF THE TUBE, (C) A LOAD RESISTOR IN THE ANODE CIRCUIT OF THE TUBE, THE ANODE OF THE TUBE BEING CONNECTED TO GROUND THROUGH THE LOAD RESISTOR, (D) A NON-INVERTING AMPLIFIER CONNECTED TO RECEIVE AN INPUT FROM THE ANODE OF THE TUBE, (E) A FIRST TRIODE CONNECTED TO RECEIVE THE OUTPUT OF THE AMPLIFIER ON ITS GRID, (F) A LOAD RESISTOR IN THE PLATE CIRCUIT OF THE FIRST TRIODE, CONNECTING THE PLATE OF THE FIRST TRIODE TO THE POSITIVE TERMINAL OF THE HIGH VOLTAGE POWER SUPPLY, (G) A SECOND TRIODE HAVING ITS PLATE CONNECTED TO THE POSITIVE TERMINAL OF THE POWER SUPPLY AND ITS GRID CONNECTED TO THE PLATE OF THE FIRST TRIODE, AND 